This disclosure relates to the field of nuclear magnetic resonance (NMR) well logging apparatus and methods. More specifically, the disclosure relates to methods and apparatus for NMR well logging that can make accurate measurements of NMR properties of formations surrounding a well at greater speed of motion of the well logging instrument without degradation of the measurements as a result of motion of the well logging instrument.
NMR-based measurement of the fractional volume of pore space (porosity) of subsurface formations, which is substantially independent of the rock mineral composition (matrix) of the subsurface formations, has been widely accepted. In complex environments, where the matrix properties are not well known, NMR measurements may provide more accurate porosities than other well logging instruments used to determine porosity. In addition, NMR transverse relaxation time (T2) distributions may provide additional information about bound-fluid porosities and reservoir quality. Notwithstanding the superior quality of NMR porosity measurements, NMR well logging remains a niche service. One reason that has hindered NMR well logging from becoming a more widely used service for determination of subsurface formation properties is the relatively slow logging speeds that may be used with NMR instruments known in the art compared to that of other porosity tools.
NMR-determined porosities and T2 distributions are adversely affected by speed of motion of the well logging instrument along a wellbore. Speed effects are more problematic with NMR measurements because of the fact that NMR measurements take a relatively long time (e.g., seconds) compared to other logging tool measurements. Logging speeds for NMR well logging instruments known in the art are relatively slow, e.g., typically in the range from 300 to 900 feet per hour depending on the logging environment and the specific application.
NMR well logging measurements are based on the application of a static magnetic field B0 to formations adjacent a wellbore to polarize the nuclear magnetic spins of susceptible nuclei (e.g., hydrogen) along a selected direction and applying on-resonance radio-frequency magnetic fields B1 to manipulate the magnetization (for instance by generating transverse magnetization and then repeatedly refocusing the transverse magnetization). In a well logging instrument, the spatial distributions of the two types of magnetic fields B0 and B1, are optimized to localize a “sensitive region”, i.e., a region where nuclear magnetic resonance is induced in the susceptible nuclei, in the formation. Many types of well logging instruments are moved along the wellbore while making measurements of selected formation properties. Motion of an NMR well logging instrument relative to the formation affects the NMR measurements because both the static (B0) and RF (B1) magnetic fields will have time dependence. These effects can be modeled in detail by using the Bloch equation with the relevant time dependent fields.
To obtain a general understanding of logging instrument motion effects on NMR measurements, it is useful to categorize the motion effects into three distinct effects:
1. Incomplete polarization;
2. Enhanced signal decay; and
3. Spoiling of longitudinal magnetization prior to measurement.
To perform a quantitative NMR measurement, the nuclear spins have to be first polarized to a known longitudinal magnetization, such as the thermal magnetization. RF magnetic field pulses are then applied to convert the longitudinal magnetization to transverse magnetization. The transverse magnetization induces RF energy that can be measured, for example, by inductive detection in an antenna. Ideally, the initial polarization is obtained by exposing the formation to a static magnetic field of strength B0 for a time that is relatively long with respect to the longitudinal relaxation time, T1 This way, the nuclear magnetic spins reach thermal equilibrium where the magnetization is exactly proportional to the formation porosity and the hydrogen index, with a known proportionality factor that depends only on the strength of the static magnetic field B0 and the temperature T. With a short-length well logging instrument that is moving along the wellbore, these condition can become difficult to fulfill when the values of T1 become longer than a few seconds.
This problem has been addressed in one aspect by adding a long prepolarization magnet in front of the measurement section, as described, for example in U.S. Pat. No. 6,140,818, “NMR logging tool and method for fast logging”, issued at Oct. 31, 2000.
As long as the prepolarization section (i.e., a permanent magnet or DC electromagnet) is long compared to vT1, for even the longest expected longitudinal relaxation time, all nuclear magnetic spins are fully polarized. Here v is the instrument speed during measurement.
The NMR signal induced after applying the RF pulses is generally designed to be proportional to the initial longitudinal magnetization. For a quantitative measurement, it is important that RF magnetic field pulses do not perturb the longitudinal magnetization before the intended measurement. This has led to the concept of non-overlapping measurements, also disclosed in the above mentioned '818 patent.
In a simple implementation, the RF magnetic field pulse sequence consists of an initial 90° reorienting pulse followed by a selected length “string” of 180° refocusing pulses. The RF magnetic field pulses may be induced by applying selected duration and amplitude RF electric current at the Larmor frequency of the susceptible nuclei in the sensitive region through an antenna. The same antenna may be connected to receiving circuits between successive RF pulses to detect the amplitude of nuclear magnetization spin echoes. In some implementations a separate antenna may be used for detecting the NMR signals. Such sequence is known as a Carr-Purcell-Meiboom-Gill (CPMG) RF pulse sequence In this case, it may be assumed that the duration of the CPMG pulse sequence is no longer than Lant/v, i.e., the duration required for the logging instrument to move the length of the antenna. The initial 90° RF magnetic field pulse tips the longitudinal magnetization into the transverse plane and the subsequent string of 180° RF magnetic field pulses act to refocus the nuclear magnetic spins to keep the transverse magnetization coherent. As the instrument and its antenna(s) move upwardly, the spins in the formation below the bottom of the antenna(s) do not experience the 180° refocusing pulses anymore, and the magnetization decays. Tmeas may denote the duration after the initial 90° RF magnetic field pulse that the formation experiences the 180° refocusing pulses. This is the effective measurement time. To a first order, Tmeas is given by Tmeas=(x/Lant) (v/Lant) where x is the distance along the formation from the position where the bottom of the antenna was disposed at the time of the 90° RF magnetic field pulse. This assumes that the duration of the CPMG pulse sequence is 1/(v/Lant). Tmeas sets the limit of the longest transverse T2 relaxation time that can be accurately determined.
As the logging instrument moves upwardly, the 180° refocusing pulses start to act on nuclear magnetic spins of the formation that have not been affected by the initial 90° pulse of the CPMG sequence. Consequently, the magnetization of these spins will be strongly perturbed and they will deviate from the thermal equilibrium, but they will not generate a coherent NMR signal. This region of disturbed magnetization is known as a “hole-burning” region.
In order to be able to start a subsequent CPMG sequence, the instrument has to move a distance Lant after the end of the current CPMG sequence in order to encounter spins in the formation that have not been disturbed yet from equilibrium by the RF magnetic field pulses. As a consequence, no NMR information can be obtained from this region and the porosity sensitivity of the instrument to this region is essentially zero.
A simple modification that can increase the fraction of the formation which is measured by the NMR instrument with high porosity sensitivity may be obtained by decreasing the duration of the CPMG sequence (i.e., decreasing the number of 180° refocusing pulses). One may reduce the duration of the CPMG pulse sequence by a factor of ε. This has the advantage that the length of the hole-burning region is reduced from Lant to around εLant. Therefore, the fraction of the formation that can be investigated increases from about 50% to around 1/(1+ε). However, this comes at a cost of reduced effective measurement time that is now a maximum of only εLant/v. This greatly limits the T2 resolution as compared to the standard implementation described above. The fraction of time that RF refocusing magnetic field pulses are applied to the formation decreases from 50% to ε/(1+ε).
One possible solution to increase the porosity sensitivity over the formation without reducing the effective measurement time is based on multi-frequency operation of the NMR instrument. The carrier frequency f of subsequent CPMG sequences is systematically switched between at least two frequencies, e.g. from f1 to f2. Changing the frequency changes the depth of investigation (DOI) of the sensitive region based on the Larmor condition B0(r=DOI)=2πf/γ. By switching the DOI, the perturbed region from the previous CPMG measurement sequence can be avoided. With this approach, most of the formation along the borehole can be investigated. It requires that the difference between RF frequencies (and therefore DOI) is large enough to avoid any interference. The systematic variation of DOI can complicate the interpretation when invasion or formation damage is present. This approach also leads to a systematic variation in signal to noise ratio because it will depend on DOI. Note that the effective measurement time Tmeas still varies across the formation.
Another possible solution to overcome the motion effects for NMR measurements was proposed by Kruspe et al., U.S. Pat. No. 6,637,524, “Non-rotating sensor assembly for measurement-while-drilling application”, Oct. 28, 2003. In this patent, it was suggested to build a logging device where the NMR sensor is built on a slidable sleeve that is mechanically separate from the rest of the bottom-hole assembly. This allows in principle to keep the NMR sensor temporally fixed relative to the formation and conduct an NMR measurement. After the measurement is complete, the NMR sensor is then repositioned to a different part of the formation that has not been affected by the RF pulses of the previous measurement sequence and the process is repeated. This approach requires a complicated mechanical design and is therefore associated with high cost and low reliability. The method and apparatus described in the '524 patent does not appear to have been developed successfully for commercial services.